Mobile communication involves signals being sent between a mobile station (MS) and a transceiver that can provide an interface for the MS to communicate to and from other network resources, such as telecommunication networks, the Internet, and the like, to carry voice and data communications, possibly also location-finding features. The transceiver might be a component in a base transceiver station (BTS) that handles traffic from multiple transceivers. The BTS might also include antennas and encryption/decryption elements. The antennas might be selective antennas, wherein different MSs at different locations might communicate to their respective transceivers via different antennas of the BTS. The BTS may have a wired, wireless, and/or optical channel to communicate with those other network resources. A BTS might support one or more transceivers and a given base station for supporting mobile communication might have a base station controller (BSC) that controls one or more BTS of that base station.
Examples of mobile stations include mobile phones, cellular phones, smartphones, and other devices equipped to communicate with a particular BTS. While herein the mobile stations are referred to by that name, it should be understood that an operation, function or characteristic of a mobile station might also be that of a station that is effectively or functionally a mobile station, but is not at present mobile. In some examples, the mobile station might be considered instead a portable station that can be moved from place to place but in operation is stationary, such as a laptop computer with several connected peripherals and having a cellular connection, or the mobile station might be stationary, such as a cellular device that is embedded in a mounted home security system. All that is required is that the mobile station be able to, or be configured to, communicate using a mobile communication infrastructure.
A BTS might be controlled by a parent BSC via a base station control function (BCF). Each of these elements could be implemented using hardware and/or software and include network management and maintenance functionality, but a base station can be described as having one or more transceiver that communicate with mobile stations according to an agreed-upon protocol. This can be by having the BTS being configured, adapted or programmed to operate according to the agreed-upon protocol for a BTS and having the MS being configured, adapted or programmed to operate according to the agreed-upon protocol for a MS. The protocol might include details of how to send data between a transceiver and a MS, how to handle errors, how to handle encryption, and how to send control instructions and status data between the BTS and the MSs. For example, parts of the protocol might include interactions wherein an MS contacts a BTS and the BTS indicates to the MS what timing, carrier frequency, and other protocol options the MS is to use. This interaction might include carrying voice data, carrying text data, carrying other data, providing for intracell handover and other tasks.
For simplicity of explanation, in many examples herein, communications is described as being between a BTS and a MS for interactions with one MS, but it should be understood that the interactions might be from a BTS to a transceiver, to a radio circuit, to an antenna, to a MS antenna, an MS radio circuit, to software/hardware in the MS, and a corresponding path in the other direction from the MS to the BTS. Thus, in some examples where a BTS is communicating with an MS, it is via a transceiver and the example ignores mention of the other transceivers that the BTS might be controlling.
Examples of protocols that a BTS might use includes GSM (Global System for Mobile Communications; trademarked by the GSM Association) 2G+ protocols with Gaussian minimum-shift keying (GMSK), EDGE protocols with GMSK and 8-PSK keying. A BTS might handle multiple transceivers that use multiple sets of carrier frequencies within a spectrum band of wireless spectrum that the protocol allows for. Thus, where a spectrum band is logically divided into carrier frequency spectra, a transceiver might use channels that use one (or more) of those carrier frequencies to communicate with an MS. The protocol might specify that for a given channel, there is an uplink subchannel and a downlink subchannel, possibly separated in carrier frequency from each other. In some cases, the uplink subchannel has a carrier frequency adjacent to that of the downlink subchannel. In some cases, all the uplink subchannels are in one spectrum band and all the downlink subchannels are in another spectrum band. For ease of explanation, sometimes a channel is described as having an uplink portion and a downlink portion as if it were one channel, even if the portions are widely separated in carrier frequency.
Some BTSs might provide for frequency hopping, where the transceivers and the mobile stations rapidly jump together from carrier frequency to carrier frequency to improve overall BTS performance. The protocol might specify the hopping sequences to use.
In the GSM protocol, transceiver-MS communication involves frames and each frame has up to eight timeslots. With eight timeslots, a transceiver sends out a frame that is directed at up to eight MSs, with each MS assigned a unique timeslot in the frame by the transceiver's BTS. The MSs can send their transmissions in their allotted timeslot and since each MS that is communicating with that transceiver knows which timeslot they are to use, similarly situated MSs can communicate back to the transceiver in their allotted timeslot. A transceiver might not use all eight timeslots.
A signaling channel, such as the GSM protocol's Common Control Channel (CCCH) might be used to convey to the MSs what their allocations are for timeslots and carrier frequencies. For example, some Common Control Channels are used to make access requests (e.g., making RACH requests, which are from a MS to a BTS), for paging (e.g., making PCH requests, which are from a BTS to a MS), for access grant (e.g., an AGCH, which is from a BTS to a MS), and cell broadcast (e.g., CBCH, which is from a BTS to a MS). The AGCH (Access Grant Channel) is used for granting timeslot allocations/carrier allocations. Another channel, the Broadcast Control Channel (BCCH) might or might not be used to send information to the MS, such as Location Area Identity (LAI), a list of neighboring cells that should be monitored by the MS, a list of frequencies used in the cell, cell identity, power control indicator, whether DTX is permitted, and access control (i.e., emergency calls, call barring, etc.).
Examples of BTSs include cellular telephone towers, macro-cell transceivers, femto-cell transceivers, picocells (which might have only one transceiver) and the like. BTSs will communicate with MSs wirelessly. Some BTSs have a backhaul (the interface between the BTS and the other network resources) that is wired, such as with a cellular telephone tower, while some might have a wireless backhaul, such as a microwave point-to-point bidirectional communications channel. Thus, a BTS might be any of several different types of electrically powered devices that receives data streams from MSs and processes those and/or forwards them to other network resources, as well as receiving data streams from the other network resources and processing those and/or forwarding them to MSs over the BTS-MS link(s). In this sense, a BTS acts as an access point for the MSs, to allow an MS to access network resources such as a telecommunications network, the Internet, private networks, etc. The access might be used to route voice calls, other calls, texting, data transfer, video, etc.
A telecommunications network behind a BTS might include a network and switching subsystem that determines how to route data to an appropriate BTS and how to route data received from a BTS. The telecommunications network might also have infrastructure to handle circuit connections and packet-based Internet connections, as well as network maintenance support. In any case, the BTS might be configured to use some protocols with MSs and other protocols with the backhaul.
The protocols for communication between MSs and BTSs might be such that they are standardized so that any standard MS can communicate with any BTS, assuming range requirements are met and membership requirements are met (e.g., that the MS has identified itself to the BTS in such a manner that the BTS, or a service that the BTS uses, determines that the MS is a member of an authorized group or otherwise authorized to use the services provided by the BTS. Some example protocols include the GSM protocols, sometimes referred to as 2G (i.e., second generation) network protocols. Other examples include GPRS (General Packet Radio Services), EDGE (Enhanced Data rates for GSM Evolution, or EGPRS), 3G (third-generation 3G UMTS standards developed by the 3GPP body, or fourth-generation (4G) LTE Advanced protocols.
In these protocols, there are rules for spectrum band use, timing, encoding and conflict resolution. As a BTS is likely to have to communicate with many MSs at the same time, the available wireless communication pathway is divided up according to the protocol. A given protocol might have the available wireless communication pathway divided up by frequency, time, code or more than one of those. This allows multiple users to share the same wireless communication pathway.
For example, with a Time Division Multiple Access (TDMA), the BTS and the multiple MSs agree on the division of time periods into timeslots (or “burst periods”) and where a first MS might interfere with a second MS, the first MS is assigned a first timeslot and the second MS is assigned a different timeslot of the available timeslots. Since different MSs use different timeslots (and they all agree on timing sufficiently well), they can share a common carrier frequency and their respective transmissions do not interfere. An example would be where there are eight timeslots of 576.92 μs (microseconds) each for each frame and so an MS assigned the first timeslot will perhaps transmit a number of bits during the first timeslot, stop transmitting at or before the end of its timeslot, remain silent, then during the first timeslot of the next period, continue transmitting, if desired. Similar allocations occur for a MS to determine when it is to listen for something from a BTS (and for the BTS to determine when it is to start transmitting that data).
Thus, using a single carrier frequency, each transceiver of a BTS can communicate with up to eight MSs and communications to those MSs is grouped into a TDMA frame and transmitted on the downlink channels that use that carrier frequency channel. The timing is such that each of those MSs can communicate in their respective timeslots to the BTS on the uplink channels that use that carrier frequency channel. This is referred to as a “TDMA frame” and the data rate over all eight MSs using that carrier frequency is 270.833 kilobits/second (kbit/s), and the TDMA frame duration, in either direction, is 4.615 milliseconds (ms).
Frequency Division Multiple Access (FDMA) is another way to divide up and allocate the available wireless communication pathway. With FDMA, the spectrum bandwidth available or allocated for the wireless communication pathway is divided up into different channels by carrier frequency. A first MS might be assigned one carrier frequency and a second MS might be assigned another carrier frequency, so that both can send or receive to or from one BTS simultaneously.
In the above examples, a plurality of mobile stations communicate with a BTS perhaps simultaneously, wherein communication between the BTS and a specific MS comprises sending information in a signal from the specific MS or from the BTS such that collisions of wireless signals are avoided, by having the BTS and the specific MS agree on which timeslot of a plurality of timeslots is to be used (TDMA), and/or agree on which carrier frequency of a plurality of carrier frequencies is to be used (FDMA). These are examples of multiple-access communications.
In another type of multiple-access communication, called “Orthogonal Frequency Division Multiple Access” (OFDMA), mobile devices are assigned subsets of subcarriers, where orthogonal narrow frequency subchannels are assigned to mobile devices for more efficient use of allotted spectrum compared to FDMA.
In some frequency allocations, the allocation is per channel block, where a channel block is a set, or group, of bidirectional channels, wherein each bidirectional channel uses an uplink carrier frequency for an uplink subchannel and a downlink carrier frequency for a downlink subchannel. The channels might be grouped together into sets of two or more channels based on some logic for classification such that each set shares a common identifier or attribute.
In some protocols, the spectrum is divided into subspectra for carrier frequencies and also the periods are divided into timeslots. Typically, the BTS includes logic to determine which channels to allocate to which MSs. In assigning a channel for use by a MS, the BTS might assign a particular transceiver to use a particular carrier frequency and indicating to an MS that it is to use that particular carrier frequency and also indicate which timeslot to use from a frame transmitted/received using that carrier frequency. The channel might comprise an uplink subchannel and a downlink subchannel. It may be that a given transceiver-MS communication uses more than one channel, e.g., more than one carrier frequency and/or more than one timeslot, but in many examples herein, the protocol is illustrated as being with respect to a MS that uses a channel comprising just one carrier frequency and just one timeslot.
In yet another example of multiple-access communications, called “Code Division Multiple Access” (CDMA), mobile devices might use the same timeslot and carrier frequency, but each mobile device is assigned a unique pseudorandom code to encode the signals to and from the BTS such that even when MSs simultaneously transmit using the same carrier frequency, or almost the same time, and/or the same timeslots, if those are used, applying the unique CMDA code allows for multiple transmitters to occupy the same time and frequency, as the receivers can separate out different receptions by decoding using the pseudorandom codes to decode each specific signal well enough for demodulation.
In effect, CDMA separates the channels not strictly by time or strictly by frequency. The use of CDMA results in a transmission of spread-spectrum signals, spread across a larger bandwidth than without encoding, by using a chipping rate that is faster than the signal bit rate. Thus, encoding signals with pseudorandom codes can replace the timing and frequency elements typically found in TDMA/FDMA protocols, as each code represents some element of articulation in both the time and frequency domain. In CDMA communications, signal propagation delay and timing between the MS and the BTS is understood and so the pseudo-random code is applied to a received signal across some number of bits/chips which, of course, occupy both some discretized span of the time domain and some discretized span of the frequency domain.
In some multiple-access protocols, more than one approach is used.
In GSM protocol digital mobile radiotelephone systems, MSs and BTSs leverage communications across both frequency and time division multiple access (FDMA/TDMA) channels such that MSs can share the same transmit and receive carriers via the assignment of distinct timeslots over each carrier frequency and each carrier frequency might be handled by a distinct transceiver or transceiver module or logic block.
In GSM, the BTS is responsible for assigning a timeslot to the mobile station (MS) when it requests access. In a GSM frame structure, there are eight timeslots within each TDMA frame. The number of carrier frequencies used can vary. In some regions, some carriers are licensed for a large number of carrier frequencies and MSs in those regions are configured to accept instructions to use one of as many as a thousand carrier frequencies (which a BTS would also support). For instance, in Europe the GSM 900 MHz spectrum band comprises 25 MHz of spectrum. If this is logically allocated into 200 kHz carrier frequencies (e.g., a carrier frequency centered within each 200 kHz subspectra band), and transceivers send signals on those carrier frequencies, this provides for 125 carrier frequencies. The use of guard bands (unused carrier frequencies) in the frequency domain might reduce this number, but might provide added reliability or ease of signal processing. Where a TDMA frame allows for eight timeslots, a BTS having sufficient numbers of logical or actual transceivers available, could support 8*125=1000 MSs channels simultaneously. With time division and frequency division, there can be guard slots and guard frequencies, respectively, so that one division has some separation from an adjacent division. With some protocols, more than one timeslot and/or more than one carrier frequency can be assigned to one MS, to provide greater bandwidth.
In some cases, there are multiple BTSs within range of supported MSs and so the support of the MSs can be spread among the BTSs and perhaps they coordinate so that adjacent BTSs avoid using the same carrier frequencies when possible. BTSs might be programmed to spread these frequencies across their towers with a specific re-use scheme. It might also be that a BTS is limited in the number of MSs it can support by the size of the pipe to the other network resources. In one example, a BTS uses from 1 to 15 carrier frequencies (i.e., its transceivers transmit using 1 to 15 carrier frequencies in sending/receiving frames, so it could support anywhere from 8 to 120 simultaneous users.
Each MS typically includes a processor, memory, radio circuitry, a power source, display, input elements and the like to perform its functions. The processor might read from program memory to perform desired functions. For example, the program memory might have instructions for how to form a data stream, how to pass that to the radio circuitry, how to read an internal clock to determine the value of a system clock to appropriately time listening and sending, and how to set appropriate frequencies for transmissions and reception.
Each BTS typically includes a processor, memory, radio circuitry, power source(s), interfaces to the telecommunications network, diagnostic interfaces and the like to perform its functions. The BTS processor might read from program memory to perform desired functions. For example, the program memory might have instructions for how to form a data stream, how to pass that to the radio circuitry, how to communicate with the telecommunications network, how to read an internal clock to determine the value of a system clock to appropriately time listening and sending, how to set appropriate frequencies for transmissions and reception, how to keep track of the various MSs and their state, location, allocation, etc. and perhaps store that into locally available memory.
In the manner described above, an MS will contact a BTS to get allocated some timeslots in frames in some carrier frequencies and the BTS will inform the MS of the MS's allocation. As both the BTS and the MS have the same system clock (or approximately so), they will communicate within their allotted timeslots and carrier frequencies. The assignment and communication of the assignments to the MSs might occur using a random access channel that is used by the MS to request an allocation. In the GSM protocol, this is referred to as a RACH process.
In the example of GSM, communication over the wireless communication pathway is parsed into TDMA frames of duration 4.61538 ms, with eight timeslots per TDMA frame. Each timeslot is long enough to hold 156.25 bits of data. In one application, the MS or BTS will transmit 148 bits of data in a timeslot, over 546.46 μs, with 8.25 bits (30.46 μs) of a guard time between timeslots. In the GSM900 Band, the wireless communication pathway has a bandwidth of 25 MHz in the uplink and downlink directions each, using the spectrum band of 890-915 MHz for uplink subchannels and the spectrum band of 935-960 MHz for downlink subchannels, providing for 125 carrier frequencies (125 carrier frequencies in each direction, spaced 200 kHz apart). With 200 kHz of guard separation on each side of each spectrum band, that leaves 24.6 MHz of spectrum, or 123 carrier frequencies, for moving data. The total capacity of such a wireless communication pathway (in both directions) would then be 156.25 bits per timeslot times eight timeslots per frame times 216.667 frames/second*123 carriers=33.312 Mbits/second.
Given that the MSs can be mobile, they might be some distance from the BTS and that distance might be changing, such as where the MSs is being used to carry on a voice conversation over the telecommunications network while the BTS is fixed to a cellphone tower but the MS is 10 km away and moving at 100 KPH. If the BTS and MS are within a few meters of each other and the MS is not moving, the propagation time of the signals and the Doppler shift due to movement can be ignored. If the MS is moving 100 KPH relative to the BTS, perhaps that can be ignored, but if the MS is some distance away, the propagation time needs to be taken into account or else transmissions in one timeslot will not be received entirely within that timeslot but might arrive late, in the time of another timeslot, which could cause communications losses.
To account for propagation delays, a transmitter will advance or retard the transmission and send bursts of radio frequency (RF) signals to account for propagation delays and a receiver will expect an allocated transmission at an adjusted time. Where there are many MSs and one BTS it is often useful for the MSs to be the ones adjusting their transmission times, so that the place where the timeslots are all aligned is at the BTS. Likewise, the BTS can send its transmissions in the designated timeslots, but the MSs will delay or advance the time at which they listen or expect to receive a transmission, to account for propagation delays. It may be that in addition to the BTS allocating a timeslot or slots and a carrier frequency or carrier frequencies to an MS, the BTS will indicate to the MS what the propagation delay or distance is between the BTS-MS.
For a BTS operating using the GSM protocol, the BTS will know the propagation delay of a MS signal because of how the signal arrives on the RACH (Random Access Control Channel). The RACH channel is an uplink-only timeslot that is used when a MS needs to access a channel to send data. The MS will request channel access by sending a signal burst that is 87 bits long on the RACH. The RACH burst is designed so that there are 69.25 bits of guard period between it and the next timeslot. As a result, the burst can slide within the RACH slot by up to 69.25 bits without ill effects. When the RACH burst arrives at the BTS, the BTS can measure how many of these guard bits the signal burst slipped to the right (i.e., moved out further in time) and thus it can determine the signal's propagation delay. When the BTS responds to the MS with information about its channel assignment, the BTS will include what is called a “timing advance” (TA), which might be expressed as a number of bits that the MS should advance its signal by in order arrive at the BTS within the correct timeslot and not bleed into an adjacent timeslot. In the GSM protocol, the timing advance value can be anywhere between 0 to 63 bits, where 0 bits corresponds to no round-trip propagation delay and 63 bits corresponds to the propagation delay that would be experienced with a MS that is 35 km away from the BTS with the wireless signals traveling at the speed of light.
Without careful timing, transmissions from MSs operating at different distances can arrive at the BTS within the same timeslot and cause collision or overlap. These collisions create interference from the perspective of the BTS, which disrupts the quality and reliability of communications. Guard time (measured in bits and referred to as “guard bits”) can be employed to prevent burst timing errors from creating signal collisions, but this can only account for small time alignment errors in internal clocks and cannot account for differences in extended and variable propagation distances.
For example, there might be 30.461 μs of guard time (8.25 guard bits) between timeslots, so that even if a first MS was 4.569 km away from the BTS (9.138 km in round-trip distance) and assigned a first timeslot and a second MS was right near the BTS and assigned the next timeslot, the relative propagation delays of the signals would not result in interference. This is because while the signal from the first MS would be delayed by 30.461 μs, the BTS would receive the later part of the transmission during the guard time, and that transmission would end before the second MS's timeslot began. Often, the guard time is too short to accommodate MSs at all distances they might be found at. For example, if a MS is 10 km (20 km round-trip) away, the propagation delay of a transmission from that MS to the BTS would be delayed by 33.333 μs, which is more than the guard time, so the BTS would be receiving that transmission at the same time as a transmission from another MS that has been assigned the next timeslot.
One solution to accommodate distant MSs sharing the same BTS is to use a timing advance mechanism. The GSM protocol provides for an example of this. In the initial handshake between the MS and the BTS, such as the GSM protocol's uses of a Random Access Channel (RACH) communication, the BTS determines a distance between the MS and the BTS. The BTS might transmit and receive timestamps during a RACH handshake in calculating a distance between the MS and the BTS for each MS is based on an uplink propagation delay.
The determined distance might not be the actual distance between the MS and the BTS, but for many purposes, a pseudo distance is sufficient. As used herein a “pseudo distance” is a value that might or might not be an actual value for a distance, but it is used as a proxy or as the deemed distance, i.e., a module in the MS, the BTS, or elsewhere will assume that value to be the distance and the various components are designed such that using that value works sufficiently well when that value is sufficiently close to the actual value. As an extreme example, suppose an MS and a BTS are 2 meters apart, but there is something in between them that prevents a direct signal and the closest path is a 3 km path that involves numerous reflections. In such a case, the pseudo distance would be 3 km and the MS and BTs would operate assuming that they are separated by 3 km. Since the signal path that their transmissions follow is 3 km, using that as the value for the distance between them works.
In general, a pseudo distance, or pseudo range of distances, that is measured between two objects might differ from their actual distance or range of distances might be measured by determining the time it takes for a radio frequency signal to propagate from one object to the other. Due to signal reflection and multipath, the line of sight distance (or range of distances) between the origin of a signal and its recipient can be slightly different from the propagation distance of that signal, in which case the pseudo distance (or range of pseudo distances) varies from the actual distance (or range of distances). But with consistent use, many operations can work with just value of the pseudo distance. In other uses, “pseudo” might be similarly used to indicate an estimate, assumed, approximate, etc. value.
Once the BTS determines the pseudo distance for an MS, the BTS stores a pseudo distance in a table that the BTS maintains for parameters and variables for each of the active MSs using the transceivers of that BTS. The BTS will communicate that value to the MS in a control message as is described elsewhere herein. The MS then is programmed to implement a “timing advance” wherein the MS considers its copy of the system clock, subtracts a propagation delay corresponding to the pseudo distance and sends its transmission to the BTS earlier than the start of its scheduled timeslot. A RACH process might include various steps as described in further detail below to determine these values.
As used herein, a propagation delay can be calculated from a propagation distance and vice versa, using c=3*108 m/s as the conversion factor or an approximation thereto. Where there is a standardized bit rate for transmissions, such as 270.833 kbits/s for GSM, the propagation delay or distance can be expressed as a number of bits. For example, a 12 km separation would result in a round-trip propagation delay of 80 μs and with each bit being transmitted over 3.692 μs, the 12 km separation and the propagation delay of 80 μs could be equivalently represented as being a separation or propagation of 22 bits (21.66 to be more precise). Thus, one “bit” of propagation would be equivalent to around 555 meters of round-trip propagation distance and 3.692 μs.
MSs operating at different distances from the BTS will be assigned different timing advances to accommodate their respective communication distances. For convenience, this might be expressed as an integer number of bits. To account for MS movement, this timing advance value, which is communicated to the MS and is used by modules in the MS to determine when to transmit or receive, might be updated periodically and frequently enough to accommodate moving targets that might have a time-variant communication distances relative to the BTS. For example, where a user is using a MS on a high-speed train traveling at 200 KPH, the distance might need to be updated more frequently than if the user is walking on a street.
In the specific example of the GSM protocol, the timing advance is represented as a 6-bit value where the minimum value represents a 0-bit timing advance and the maximum value represents a 63-bit timing advance. Since each bit in the GSM protocol is assumed to correspond to 3.692 μs (and about 555 meters in round-trip propagation delay), 63 bits of timing advance would be used where the pseudo distance is around 555 m/bit*63 bits=34,965 m, or about 35 km. Thus, this timing advance approach would work fine for MSs that are between 0 and 35 km from the BTS. In the GSM protocol, BTS are programmed to, or at least expected to, not respond to requests from a MS if the BTS determines that the MS is further than 35 km from the BTS. This is not a problem when there are other closer BTSs or a distribution of BTSs where all points are within 35 km of one or more BTS.
With a timing advance, the MS sends a transmission before its timeslot begins (from the MS's clock timing) and when it is received at the BTS after a propagation delay, the BTS receives it entirely within its timeslot when the timing advance corresponds to the propagation delay. The MS can correctly do this, because it has been provided a value for how much of a timing advance to use. Note that the actual distance, and therefore the actual propagation delay, might vary from the pseudo distance, but that is often not a problem since the MS-BTS communication has some leeway that is there to handle internal clock differences, transmitter variances, etc.
That timing mechanism works well when there is always one or more BTS within 35 km of any MS, but this might not always be the case. In some geographic regions, it might not be practical, feasible, or economical to have BTSs no more than 35 km from any point in the region. For example, in rural, remote, or island geographic regions, BTS infrastructure with such spacing might leave BTSs unused or unable to be installed or obtain electrical power, as the terrain might be inaccessible and users with MSs might be sparse and widespread. In such situations, an “extended range” mechanism might be used. The GSM protocol allows for such a mechanism.
With an extended range mechanism, each MS is assigned two consecutive timeslots instead of one, so an MS can communicate with a BTS without needing any timing advance, as the transmission can be delayed at the BTS by as much as the duration of one timeslot. While this increases allowed MS-BTS distance (e.g., from 35 km to 120 km), it decreases throughput by half, as there would be only four assignable timeslots available in each TDMA frame instead of eight. This might not be a concern in rural, remote, or island areas, if data rates are low. By using a combination of the timing advance mechanism and the extended range mechanism, the maximum allowed MS-BTS could be 35 km+85 km =120 km.
With the extended range mechanism, each MS is allotted an entire timeslot as an additional guard period, which reduces the throughput by half. A variation of this is the “sorted extended range mechanism” similar to that shown in, for example, U.S. Pat. No. 5,642,355. With the sorted extended range mechanism, timeslots are “consumed” to be used as guard bits, but the timeslots are assigned to MSs by distance, with the closest MS getting the first timeslot and the furthest MS getting the last timeslot that is allotted to an MS, i.e., before any “consumed” timeslots that are not assigned to any MSs. The consumed timeslots are used for guard bits that are needed since the extended range of the MSs will spread out the transmissions. In effect, this “divides up” unused timeslots between the bursts.
If there is more than an 85 km gap, or for other reasons, a “ring extended range” mechanism might be used. With the ring extended range mechanism, a fixed minimum distance is assumed, the timing at the BTS is adjusted by that fixed minimum distance, and a MS that is closer than the minimum communication distance is not supported, as the BTS assumes that all MSs are at least that distance away. This is similar to the approach shown in U.S. Pat. No. 6,101,177. The 35 km range obtained using the timing advance mechanism can be used to support a MS-BTS distance that ranges from the minimum distance to the minimum distance plus 35 km without requiring any MS modifications. In one example, the minimum distance is 85 km, but a different minimum communication distance might be used. In that example, then, the BTS could support an MS that is between 85 km and 120 km from the BTS.
The ring extended range mechanism can be used with 8 of 8 timeslots allocated and can handle MSs with distances that range from 85 km to 120 km from the BTS. However, this creates a physical coverage gap some radius away from the BTS because any signal burst sent from that area will arrive at the BTS too early relative to how the BTS views its timeslots. Instead, the BTS provides coverage for a ring of area. The ring extended range mechanism might be used in geographic areas that have physical gaps, such as lake or valleys, between the BTS and the MSs that it is designed to service, so it would not be a problem to have a region inside the ring where no MSs are supported.
It should be noted that the GSM system employs a TDMA frame offset between uplink and downlink subchannels. In a typical GSM frame structure, the uplink TDMA frame (or MS Tx and BTS Rx) is offset from the Downlink TDMA frame (or BTS Tx and MS Rx) by three timeslots for the purpose of ensuring that the MS does not need to transmit and receive at the same time. It will be clear to those skilled in the art of TDMA communications that this offset between uplink and downlink subchannels is independent of communications over extended distances and is not the same as the timeslot synchronization offset used on the uplink TDMA frame only in the ring extended range mechanism.
If the ring extended range mechanism is combined with the extended range mechanism, this can be used, alone or in combination, to have a BTS coverage that might be over a 120 km radius. These techniques are often sufficient for terrestrial communications, as such communications are typically limited by Earth curvature. For example, to provide line of sight communications between a ground-based MS and a BTS transceiver D distance away, the BTS transceiver should be mounted at a height of at least h=[SQRT(6370{circumflex over ( )}2+D{circumflex over ( )}2)−6370] km. For D=120 km, h=1130 m. As 1,130 meters is higher than any structure built today, tower height is much more of a limiting factor for terrestrial communications than distance and so techniques for extending distance further than say 120 km are not that useful for terrestrial communications for cellular voice, data, text, and similar capabilities, except possibly in selected locations where there are large geologic structures upon which to mount transceivers.
For regions where it is not practical to have base station towers distributed so that there is broad coverage, such as where it is impractical to locate a base station anywhere near some locations, such as within 35 km or 85 km from some location or 120 km where elevated towers can be mounted, satellite communications might be used. Typically, satellite communications is very expensive and thus only used in applications that support the costs, such as resource exploration, explorers, search and rescue, and the like.
Herein, “satellite” refers to an artificial satellite that is launched from Earth with a goal of operating in orbit and/or that operates in orbit whether assembled in whole or part on the ground and/or assembled in whole or part in orbit. A satellite might be assembled and/or operate in one orbit and move to another orbit. The satellite might be propelled or be operating without its own means of propulsion and might or might not rely on other objects in orbit to provide propulsion. As used herein, a satellite when in operation in orbit and not under propulsion is in an orbit that is more or less stable. Such orbits have a minimum distance above the surface of the Earth due to atmospheric drag. There is not a strict dividing line between sufficient vacuum to allow for orbiting and excessive atmosphere that would cause the deorbiting of a satellite, by Low Earth Orbit (LEO) of around 400 to 500 km above the Earth have been shown to be practical, but could be even lower than those altitudes for particularly dense spacecraft such as nanosatellites.
The minimum distance for a practical orbit being so large has traditionally meant that entirely different technologies were employed in satellite communications. In some cases, the ground stations were not mobile and in other cases, they were mobile but required power-intensive, heavy, large and specialized equipment. In addition to distance, movement of the satellite in orbit had to be addressed.
There are many solutions for communications between satellites and ground-based portable handsets on Earth that use that TDMA protocols for communications. Some satellite providers include Iridium™, Globalstar™, Thuraya™, and Inmarsat™ satellite systems, which are based on a uniquely developed satellite phone or user terminal (i.e., a unique hardware device that attaches or connects to an existing mobile phone by a physical or RF connection). With a specific user terminal, the design of the system, the satellite and the terminal can be simplified, as each can be designed specifically to work with the others. The downside is that it requires specific terminal equipment, which would be needed for every end user or small groups of end users, which can be cost-prohibitive and unwieldy. While the custom terminal approach simplifies the system design, as the operator is free to set the details for communication methods, power levels, frequencies, and the like, this ties the users to specific providers. As a result, the end user might need to purchase a satellite phone (or a user terminal that plugs into an existing mobile phone) that costs hundreds to thousands of dollars, is large, has a cumbersome antenna, uses significant power, and has a steep monthly service subscription to operate, and may have to do this for more than one satellite provider. This has limited the appeal of the classic satellite phone market.
As an example, U.S. Pat. No. 8,538,327 describes a modification to user equipment computes a delay measure based on data indicative of the position of a satellite and data indicative of the position of the user equipment. Timing of uplink communication from the user equipment adjusts for that delay when transmitting up to the satellite. The user equipment also computes a frequency offset based on data indicative of the position and velocity of the satellite and adjusts its uplink signal frequency accordingly to account for dynamic Doppler shift in the communications system. This, of course, requires specific user equipment on the ground that is designed for satellite communications.
As another example, U.S. Patent Publication 2006/0246913 describes a method for managing propagation delay of RF signals using sub-coverage rings characterized by reduced difference in round-trip propagation delay differences. This uses a geosynchronous Earth orbit (GEO) satellite to act as a relay to connect a remote mobile station with a base station in its network. To deal with the much greater delays that a GEO satellite introduces, separate processing devices service separate sub-coverage ring, or zone, by configuring itself for that ring's/zone's range of allowable propagation delays. The link between a mobile station and a GEO satellite cannot be closed without the assistance of additional user terminal hardware for power, signal directivity, and frequency manipulation.
What is needed is an improved system for satellite-based communications with portable or mobile devices.